The effect of alkali and Si/Al ratio on the development of mechanical properties of metakaolin-based geopolymers (original) (raw)
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Influence of metakaolin characteristics on the mechanical properties of geopolymers
Applied Clay Science, 2013
The relationship between the properties of geopolymers and the characteristics of metakaolin samples used in their preparation has been investigated. Three commercial metakaolin samples have been characterised using 27 Al-NMR to determine the coordination number of Al (IV, V and VI), and by acid and alkali dissolution to determine the reactive Si and Al content. The setting and mechanical properties of geopolymers formed from the metakaolin samples under identical conditions are reported, using Weibull statistics to analyse strength data. Although the metakaolin samples contained different levels of five coordinated aluminium (Al (V)) the mechanical properties of the geopolymers formed were very similar. The reactive fraction of metakaolinite determined by dissolution in 8M NaOH provides the most relevant long-term indicator of geopolymer performance.
Mechanical properties of metakaolin-based geopolymers with molar ratios of Si/Al ≈ 2 and Na/Al ≈ 1
Journal of Materials Science, 2008
The mechanical properties of four different types of geopolymers, but of the same composition (Na/Al ≈ 1, Si/Al ≈ 2 molar ratio), made using a combination of precursors, were determined. The four types were: (i) sodium aluminate (NaAlO2/NaOH solution), Ludox (colloidal SiO2 solution) and metakaolin (MK), (SAGP), (ii) NaOH, fumed silica and MK (FSGP), (iii) Ludox, NaOH and MK (LGP) and (iv) commercial sodium silicate and MK (SGP). The highest crushing strength (CCS) value obtained was for SGP (70 MPa) and the lowest value was for SAGP (16 MPa). The highest modulus of rupture (MOR) value obtained was for LGP (9 MPa) and the lowest value was for SAGP (3 MPa). The fracture toughness (K1c) and Young’s modulus (E) showed the same trend. The effect of adding sand (40 wt%) on their mechanical properties was also determined. The K1c values increased up to 65% and E values increased up to 80% compared to samples free of sand. However, CCS and MOR values did not change much and gave mixed results. Overall, porosity is found to be the chief microstructural variable limiting the mechanical properties of the geopolymers. The properties of the geopolymers are compared with those of ordinary Portland cement.
Effect of molar ratios on strength, microstructure & embodied energy of metakaolin geopolymer
2021
In this study, twenty-five geopolymer (GP) mixes were prepared by varying the alkaline solids to Metakaolin (MK) and sodium silicate to NaOH ratios from 0.1 to 0.5 and 0.2 to 1.0, respectively, thus giving a wide range of molar ratios of silica to alumina, sodium oxide to alumina and water to sodium oxide. The compressive strength of these GP mixes was determined for four curing schemes involving oven curing at 100oC for 24 h and three ambient curing with the curing ages of 3, 14, and 28 days. The test results revealed that for the manufacture of GP binder for structural applications of strength up to 90 MPa, the molar ratio of silica to alumina should be greater than 2.3, sodium oxide to alumina should be between 0.6 to 1.2, and water to sodium oxide should not exceed 12. The compressive strength of ambient cured GP mortar gets stabilized at 28 days of ambient curing. Experimental findings were also corroborated by GP microstructure analysis. The embodied energy of MK-based GP mort...
MECHANICAL PROPERTIES OF METAKAOLIN MODIFIED GEOPOLYMER AND THE EFFECT OF SI/AL RATIO
The environmental impact of the production of cement has left the construction industry with little choice but to develop an alternative material. Geopolymer synthesis has promising potential in that direction. Alkali activation of aluminosilicate raw material produces geopolymer. Flyash is a widely used precursor in geopolymer. Flyash (FA) based geopolymer is porous and hence less durable. Metakaolin is rich in aluminosilicate (52% of Sio2, 46% ofAl 2O3)and has a smaller particle size (2 µm) and high specific area (20 m 2/g)compared to flyash.Metakaolin (MK) based geopolymers are expected to give higher strength and less porous than FA based geopolymer. This research aims to investigate the use of MK as a precursor in geopolymer synthesis as a replacement for FA in various percentagesto utilize the advantages of MK over FA. MK based concrete cured at different temperaturesand various solid/ liquid ratio is tested for compressive strength. Experimental investigation of FA-MK based geopolymer in this research is found to have to have 11% to 65% less compressive strength compared to FA based geopolymer when replaced by 10% to 40% of MK. Lesser strength is attributed to the Si/Al ratio of the MK precursor.
The Open Civil Engineering Journal, 2013
This study presents a discussion on the effects of different solid-to-liquid (S/L) ratios (0.97 to 1.19) and nano-SiO 2 (NS) percentages (0% to 3%) on some properties of metakaolin (MK)-based geopolymers. The setting time and compressive strength were investigated. Mercury intrusion porosimetry, fourier transform infrared spectroscopy, and scanning electron microscopy were used to determine the microstructure of the samples. The results show that a MKbased geopolymer sample added 1% NS with the S/L ratio of 1.03 exhibits more strength and less porosity. Applying NS to the geopolymer enhances compactness and increases strength. Therefore, nanotechnology can be used to improve geopolymers.
Science of Sintering, 2020
This paper outlines the production of an inorganic polymer/geopolymer using a metakaolin by an environmental friendly, energy saving, clean technology to conserve natural environment and resources. The influence of alkali activation, i.e. different concentration of NaOH as a component of alkali activator mixture on the process of geopolymerization of metakaolin is investigated. Also, process of aging time of geopolymer is followed by several analytical methods. The structure of metakaolin and metakaolin based geopolymers and their physicochemical properties were studied using X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR) and after 28days scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was applied for the surface characterization of the samples. A shift of the Si-O or Si-O-X (X=Al, Si, Na...) bands as the molarity of activator increasing during geopolymerization process was observed by FTIR. Mass spectra of geopolymers were ...
SN applied sciences, 2022
Several studies explore optimal molar oxide ratios for metakaolin geopolymer production. However, there is not a consensus on the optimal mix, and within similar range large differences in compressive strength are reported, and consequently in the overall performance. Hence, the present work selects a specific molar oxide ratio that leads to strengths above 30 MPa (SiO 2 /Al 2 O 3 ratio of 3.19, a Na 2 O/Al 2 O 3 of 1.00, and a water-to-solids ratio of 0.52), and investigates the individual effect of the sodium-based activators (NaOH and Na 2 SiO 3) and the water content on fresh and hardened properties of metakaolin geopolymers. The tested properties include the rheology, setting time, mass loss, shrinkage, density and compressive strength. The test results show that an increase of water content (water-to-solids > 0.52) and increase of NaOH (Na 2 O/Al 2 O 3 > 1.03) have the largest impact, showing a detrimental effect on both fresh and hardened properties. Moreover, the best results are obtained when using molar ratios of SiO 2 /Al 2 O 3 at 3.14, Na 2 O/Al 2 O 3 at 0.97 and a water-to-solids ratio of 0.51, which is within the range of optimum molar ratios from previous studies. The tests are further supplemented by Thermogravimetric analysis/Differential scanning calorimetry (TGA/DSC), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Article highlights • An elevated water content can improve the fluidity of the pastes, however, it increases pore formation and detriments the strength. • A surplus of alkali content decreases the fluidity, accelerates the setting, and increases the shrinkage and surface cracking of metakaolin geopolymers. • The best performance is achieved with a water-to-solids = 0.51, and molar oxide ratios of Na 2 O/Al 2 O 3 = 0.97 and SiO 2 /Al 2 O 3 = 3.14.
Cement and Concrete Composites
A multiscale study is presented of the reaction kinetics, phase formation, mechanical properties of metakaolin-based geopolymers by varying Si/Al ratios of 1.2-2.2 and Na/Al ratios of 0.6-1.2. Macro-and nano-mechanical properties of geopolymer samples were determined by unconfined compression testing and grid nanoindentation technique, respectively. The latter, in combination with statistical deconvolution, also enables the extraction of generally 4 distinct phases together with their nanomecahnical properties and volumetric fraction within the synthesized geopolymers. Moreover, the reaction kinetics, phase formation (particularly geopolymer gel development), and mechanical property development were investigated by characterizing geopolymers cured at the final setting time, 7, and 28 days. Phase formation was characterized by Fourier transform infrared spectroscopy (FTIR) via monitoring the evolution of the Si-O-T (T: Si or Al) and Al-O bonds. Results illustrate that the fraction of geopolymer gels dominantly governs the mechanical behavior, both of which increase with the Si/Al and Na/Al molar ratios, while the final setting time increases with the Si/Al ratio, but decreases with the Na/Al ratio. The chemical composition for the best mechanical performance of the studied geopolymers is a Si/Al ratio of 1.7 and Na/Al ratio of 0.9. The relationships among geopolymer chemical compositions, geopolymer gel formation rate, and macromechanical properties are also discussed.
Construction and Building Materials, 2019
This paper explored reported the effect of sewage sludge ash (SSA) on the mechanical and microstructural properties of geopolymers based on metakaolin (MK) involving two different SiO2/Na2O molar ratios (0.8 and 1.6), two temperature curing conditions (25°C and 65°C) and various ages of curing (1, 3, 7, 14, 28, 90 or 180 days). The geopolymers tests were characterized performed using different techniques: as X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and compressive strength of mortars. Tests were performed for both high (65°C) and room (25°C) temperature curing conditions lasting for 1, 3, 7, 14, 28, 90 or 180 days. The geopolymeric samples were activated using sodium hydroxide and sodium silicate solutions using two different SiO2/Na2O molar ratios (0.8 and 1.6). The compressive strength tests showed that the replacement of MK by SSA in 10 wt.% when cured at 25 °C with the highest SiO2/Na2O molar ratio reaches similar compressive strengths after 14 days of curing compared to the samples with only MK, which reached a maximum compressive strength of 50.8 MPa at 180 days. The FTIR analyses carried out in the geopolymer pastes with SSA (10 wt.% of SSA and 90 wt.% of MK) showed a formation of N-AS -H gels in the samples cured at 25 °C. The microstructural studies by XRD, TGA and SEM pointed out the formation of a crystalline phase as Na P-type zeolite in MK/SSA based-geopolymer pastes cured at 65 °C, which explained the loss of compressive strength of the samples cured at high temperature. However, the SSA retarded the crystallization process in the MK basedgeopolymer.
Ceramics International, 2014
The present work aims to investigate the influence of the alkaline activator concentration of kaolin-based geopolymers on the compressive strength at the curing early age regarding the structural characteristics. We used alkaline activators (Na, K) at molar concentrations from 4 M to 12 M, while the volume ratio sodium silicate/MOH was kept equal to 0.20 for all compositions. Samples were consolidated by curing at 70 1C for 24 h, and the mechanical and structural characterizations were obtained after 3 days of aging. Results show that the type and the concentration of the alkali-MOH solutions changed both the amount of reacted-kaolinite, the structural characteristics of gel phases, the formation of new crystalline phases, and the compressive strengths at the early age. For all compositions, the increase of MOH concentration led to an improvement of the compressive strength at the early age, attaining 88-165% of the standard value of 4 M MOH geopolymers. Optimized compressive strengths were 25 MPa and 32 MPa for [NaOH] ¼ 10 M and [KOH] ¼12 M respectively. We propose that an optimized alkali concentration is found with an adequate charge-balance of the Si-Al tetrahedral substitution, which prevents the formation of carbonated species from the alkaline excess.